ne120 paper_tyler_bailey

Tyler A. Bailey

NE 120

12/5/2016

SRIM Calculations Applied to Ionization Chambers

I: Introduction

SRIM is a Monte Carlo code used primarily to simulate the range and stopping of ions in

matter. When used, the software takes into account the species of the ion, the energy of the ion,

the elemental composition of the target, and the phase of the target. SRIM is capable of

simulating both solid and gas targets. In spite of this, the amount of gases that can be modeled

with SRIM is limited due to lack of general theory and experimental data. Gases that can be

modeled accurately with SRIM and that are utilized for this experiment are argon, helium,

hydrogen, nitrogen, air, oxygen, and methane gas. The purpose of this project was to use SRIM

to simulate which gas would be optimal to be used inside an ionization chamber that is geared

for detecting alpha particles, is portable, and yields a sizeable voltage pulse for each incident

alpha particle. Characteristics for this optimal gas that were evaluated are the projected range of

an ion in the gas, the amount of ion pairs produced in the gas, and the energy loss of the recoiling

gas ions.

II: Ionization Chamber Fundamentals

Ionization chambers detect radiation by utilizing an electric field to separate ions from

their electrons after being ionized by incident radiation. From here the separated electrons are

usually used to form a signal in the form of voltage or current. Ions can also be used, but

electrons give a better time response since they move faster under the influence of an electric

field due to the differences in mass between electrons and ions. In general, it takes ions on the

order of milliseconds to reach the electrodes of the chamber while it takes electrons on the order

of microseconds to travel the same distance. There are two main modes of ionization chamber

operation: current mode and pulse mode. Current mode is when the electrons are converted into a

current, and the current is then manipulated and detected. This is fundamentally how standard

smoke detectors operate with an Americium-241 source. Pulse mode is when the electrons are

converted into a voltage pulse (by use of a resistor-capacitor circuit), and the voltage pulse is

then manipulated and detected. A pulse mode detector is simulated for this project. Other

characteristics of the chamber that are modeled for this project are that the chamber is operated at

standard temperature and pressure (SRIM assumes standard temperature and pressure for its

simulations), only alpha particles are simulated (ionization chambers can detect any type of

decay, however SRIM is only able to simulate alpha particles without downloading extra

packages), the source is put inside a closed chamber (this is to ensure that the incident ion

deposits its energy in the gas within the chamber), and recombination between ions and electrons

is negligible (this is true when a large enough electric field is established between chamber the

electrodes).

III.1: Stopping Power Fundamentals

One of the features that was utilized from SRIM was the ionization feature. This feature

outputs a graph and text file of ionization (in unit of eV/angstrom) versus range in matter. In

detection literature, this quantity is referred to as linear stopping power. Linear stopping power is

defined as the differential energy loss for a charged particle moving through a material divided

by the differential path length (Equation 1).

S= −ⅆ𝐸

ⅆ𝑥 (Equation 1)

The value along the trajectory of the particle is referred to as its specific energy loss. This

quantity is further described by the Bethe formula (Equation 2).

𝑆 =4𝜋ⅇ4𝑧2

𝑚0𝑣2𝑁𝐵 (Equation 2a)

𝐵 = 𝑍[ln (2𝑚𝑜𝑣2

𝐼) – ln(1 -

𝑣2

𝑐2 ) -

𝑣2

𝑐2] (Equation 2b)

N is the density of the material while Z is the atomic number of the material. 𝑚𝑜 , 𝑒 is the mass

and charge of the electron while z and v are the charge and velocity of the particle respectively. I

is the Ionization Potential of the material. The Bethe formula is applicable for both gas and solid

targets. Since this project is only concerned with gases (N is typically low for gases at standard

temperature and pressure) and alpha particle at non-relativistic velocities (this causes the second

and third term of Equation 2b to become negligible), the most important term is the atomic

number of the gas particles. The higher the atomic number of the gas particles, the larger the

specific energy loss.

Figure 1

Figure 2

Simulated ionization data for a 5.3 MeV alpha particle in air and the recoiling air ions’

ionization data are plotted in Figure 1 and Figure 2. Figure 1 is reminiscent of the Bethe

formula for this specific particle and target. For the majority of the ions trajectory, the

specific energy loss rises steadily but slowly. Towards the end of the trajectory, the particle’s

specific energy loss rises rapidly and reaches a peak (this is due to the square of the velocity

in the denominator and that the ion is effectively in the vicinity of the gas particles for a

relatively longer period of time). At the end of the trajectory, for the case of an alpha particle,

the alpha particle picks up 2 electrons becoming neutral and its specific energy loss drops to

0. From comparing the ionization values for Figure 2 to Figure 1, it is clear that the specific

energy loss due to recoiling air ions is negligible compared to the incident radiation (the

specific energy loss for the incident particles are about 2 orders of magnitude larger than the

specific energy loss for recoiling air ions). It is still imperative to ensure that the recoiling gas

ions’ specific energy loss is minimized in order to minimize the noise created from the ion

pairs formed from the recoiling gas ions.

III.2: Experimental Results

Figure 3

0.00E+00

2.00E+04

4.00E+04

6.00E+04

8.00E+04

1.00E+05

1.20E+05

1.40E+05

Ar He H2 N2 Air O2 CH4

ion

izat

ion

(eV

/mm

)

Gas

Average Ionization

Figure 4

Figure 5

Average ionization, peak ionization, and projected range for 5.3 MeV alpha particles in

different gases are displayed in Figure 3, Figure 4 and Figure 5 respectively. In Figure 3 and

Figure 4, it is clear that every gas besides helium and hydrogen have similar average

0

50000

100000

150000

200000

250000

300000

350000


Pea

k Io

niz

atio

n (

eV/m

m)

Gas

Peak Ionization

0

50

100

150

200

250


Pro

ject

ed R

ange

(m

m)

Gas

Projected Range

ionization values and peak ionization values. The low values for helium and hydrogen are

most likely due to the low atomic number of the gas particles in these two gases. In Figure 5,

it is clear that every gas besides helium and hydrogen stops these 5.3 MeV alpha particles

within 50 millimeters of traveling through the chamber. Helium and hydrogen gas both need

over 150 millimeters to stop these same particles. This depicts an inverse correlation between

peak ionization/average ionization and projected range for given alpha energy.

IV.1: W-Values and Ion Pairs

When the energy is loss by the incident alpha particle, the energy is used to ionize the gas

particles. For different types of incident particles (alphas and betas for example) and different

gases, different amounts of energy are needed to form an ion pair. These values are called W-

Values and are given in units of eV/ion-pairs. Figure 6 below displays W-Values for alpha

particles in different gases. These values were taken from Glenn Knoll’s Radiation Detection

and Measurement.

Figure 6

To determine the amount of ion pairs that are formed in each gas, the total energy loss by

the incident particle before neutralizing is needed. This can be done by taking a reiman sum

of the ionization curve like the one located in Figure 1. Similarly, a reiman sum was also

taken of the recoiling gas particles ionization curves like the one located in Figure 2. This is

done to further prove that energy loss of the recoiling gas particles is negligible. After the

reiman sum is taken, the energy loss is then divided by the W-Value for the gas. This yields

the total number of ions formed for each incident particle.

26.3

42.7

36.4 36.4 35.132.2

29.1

0

5

10

15

20

25

30

35

40

45


W-V

alu

es (

eV/i

on

pai

r)

Gas

W-Values (From Knoll)

IV.2: Experimental Results

Figure 7

Figure 8

Energy loss due to the ions and recoiling gas ions and ion pairs formed are displayed in

Figure 7 and Figure 8 respectively. From Figure 7, it is clear that the energy loss from the

5280000

5282000

5284000

5286000

5288000

5290000

5292000

5294000

5296000


Axi

s Ti

tle

Gas

Reiman Sum Energy Loss Due to Ions and Recoil

Ions Recoil

0

50000

100000

150000

200000

250000


Ion

Pai

rs F

orm

ed

Gas

Reiman Sum Ion Pairs Formed

recoiling gas ions is negligible compared to the energy loss of the incident particle. Argon

and helium recoiling ions have the lowest amount of energy loss for all the gases. From

Figure 8, it is clear that alpha particles in argon gas form the most ion pairs. This is primarily

due to the fact that argon has the smallest W-value compared to the other gases.

V.1: Conversion of Ion Pairs into Voltage

After ion pairs are separated, the electrons need to be converted into a voltage pulse in

order for signal processing to occur. This can be done by a resistor-capacitor circuit. The

electrons induce a current in the chamber’s positive electrode; this current then charges up a

capacitor to a max value. This value is dependent on the capacitance of the capacitor and the

number of ion pairs that were formed as described in Equation 3.

𝑉𝑚𝑎𝑥 = 𝑛∗1.6𝑥10−19

𝐶 (Equation 3)

In order to ensure that only the electrons are charging up the capacitor, an RC time constant is

chosen that is between the ion travel time and the electron travel time. In reality though, the

voltage will have a slightly smaller voltage peak due to geometrical characteristics of the

chamber and the initial trajectory of the ion.

V.2: Experimental Results

Figure 9

The peak voltage of a 5.3 MeV alpha particle for different gases is displayed in Figure 9.

The capacitance that was used was 100 picofarads. From this data, it is clear that argon has the

highest peak voltage. Although the voltage pulses are quite small (they are in the hundreds of

microvolts range), they can easily be amplified by a Darlington configuration transistor

(cascaded bipolar junction transistors) to get the voltage pulses in the hundreds of millivolts

range.

VI: Conclusion

Through this analysis with SRIM, it is clear that argon gas is the optimal gas for a

portable ionization chamber for the purpose of detecting alpha particles. One reason why this is

concluded is that it was simulated that 5.3 MeV alpha particles (which is in the middle of the

alpha energy range) have a projected range of less than 50 millimeters. This allows the ionization

chamber to be small and portable while still allowing alpha particles to deposit their energy in

0

0.00005

0.0001

0.00015

0.0002

0.00025

0.0003

0.00035

0.0004


Vo

ltag

e (V

)

Gas

Peak Voltage

the chamber’s gas and neutralizing before they reach the walls of the detector. Another

characteristic that was calculated for argon gas is that the recoiling argon ions deposit minimal

energy within the gas. This minimizes the voltage signal which would be interpreted as noise in

the signal processing that would occur after collection. The final characteristic that was

calculated for argon gas is that it produces the most ion pairs for given alpha energy. This will

create a larger un-amplified voltage pulse for each incident particle. This is desirable for alpha

spectroscopy purposes. The main downside of using argon gas is the cost of obtaining the gas

compared to just using ambient air. Despite this downside, if portability and large voltage pulse

is wanted for an ionization chamber, argon is the best candidate based on SRIM calculations.

VII: References

Knoll, Glenn. Radiation Detection and Measurement. John Wiley & Sons, Inc., 2000.

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